1. Technical Field
The present invention relates to a flight control device, a spacecraft, and a reference trajectory correcting method.
2. Description of the Related Art
In order to allow a spacecraft to arrive at a ground surface, a water surface, or a predetermined altitude as a target position on a celestial body having the atmosphere (e.g. planets such as the Earth and Mars, and their moons) out of the atmosphere, that is, from outer space, a reference trajectory is found during flight of the spacecraft, which is a flight trajectory serving as a reference from a current position to the target position, and the spacecraft controls its body to fly in accordance with this reference trajectory.
The reference trajectory is identified based on a range that is a distance from the current position to the target position of the spacecraft, and on velocity or energy of the spacecraft. In the atmosphere, the spacecraft changes inclination of its body so as to change lift force generated on the body, thereby changing the velocity. For example, a capsule-type spacecraft changes inclination of its body by injecting gas, and a spacecraft having wings changes inclination of its body by changing control surfaces of the wings.
The flight trajectory of the spacecraft is generated with the reference trajectory guiding method, the closed form guidance method, and real-time trajectory generation method, etc., as described below, for example.
The reference trajectory guiding method identifies the reference trajectory based on the range and the velocity of the spacecraft as shown in FIG. 7A. Specifically, if the velocity of the spacecraft becomes a predetermined landing velocity, and the range becomes zero, the spacecraft can correctly arrive at the target position.
Specifically, in the reference trajectory guiding method, a single reference trajectory as well as influence on the range (sensitivity coefficient) by variation in parameters pertain to the flight are analyzed in advance on Earth, and data indicating the analyzed results (table data) are stored on a storage device. The spacecraft flies while correcting deviation from the reference trajectory by controlling the attitude of the body using the above data so as to fly at the velocity depending on the range.
In the closed form guidance method, as shown in FIG. 7B, the reference trajectory is identified based on the drag acceleration and the velocity (or energy) of the spacecraft within a range under constraint conditions. See Jon C. Harpold; Claud A. Graves, “Shuttle program. Shuttle entry guidance”, pp. 8-9, pp. 21-24, (online), 1979, National Aeronautics and Space Administration, NASA, (searched on May 17, 2011), Internet <URL:http://ntrs.nasa.gov/search.jsp?N=0&Ntk=Report-Patent-Number&Ntt=NASA-TM-79949&Ntx=mode %20matchany&Ns=Loaded-Date|1>
Specifically, in the closed form guidance method, the reference trajectory is divided into plural phases, a reference trajectory is generated for each phase using a formula for annalistically calculating the range, and the formulas for all the phases are combined together to create the entire reference trajectory. The range is calculated using Formula (1) where R represents the range, V represents the velocity of the spacecraft, and D represents the drag acceleration.
                    R        =                  ∫                                    V              D                        ⁢                          ⅆ              V                                                          (        1        )            
It deviation occurs between the calculated range and the real range, the reference trajectory is corrected by adjusting a coefficient of an approximate formula for each phase.
In the real-time trajectory generation method, as shown in FIG. 7C, in the same manner as that in the closed form guidance method, the reference trajectory is identified based on the drag acceleration and the velocity (or energy) of the spacecraft. In the real-time trajectory generation method, the reference trajectory is calculated by calculating the behavior of the spacecraft in its flight using an equation of motion, and the range is calculated through numerical integration using Formula (1), and the reference trajectory is re-built from moment to moment using the equation of motion.